I can empathize a bit with the frustration, but DC has no incentive to age up and retire the big name heroes. It's not the fault of those characters or their fans that the Titans aren't working. They can find a way to make them matter without getting rid of the old heroes, especially as their is no endgame to progress towards as these stories are designed to be published indefinitely. And especially since those characters, despite how old they are, are always new to somebody. I'm a millennial too, and Batman was already 50+ years old when I first "met" him.
Diana in particular shouldn't be aged up and retired now of all times. If she is as ostensibly as important as the other two members of the Trinity, she and her world should be utilized more because she has a LOT of catching up to do compared to them. Retiring her and replacing her with someone like Donna Troy, who has spent more time out of her franchise than in it, wouldn't be the wisest move on DC's part. Especially not with Donna as the husk she is now.